Coastal wetlands are one of the world’s most valuable habitat types, due to the numerous ecosystem services they provide, including protection from storm surges and coastal erosion, supporting biodiversity and carbon sequestration. The soil property of waterlogging leads to slow rates of decomposition of organic matter, leading to net carbon sequestration, making these ecosystems valuable tools against climate change. The carbon storage capacity of coastal wetlands can be lost due to anthropogenic disturbances and natural causes, such as changes in land use and global warming. Accordingly, more than 50% of salt marshes and 35% of mangrove swamps have so far been lost around the world, resulting in significant losses of soil organic carbon. This study investigated the biogeochemical processes in the soils of some coastal wetland types: a mangrove swamp in Florida, USA; a salt marsh ecosystem in the UK; and a salt marsh site encroached by mangroves, also in Florida. At each site, indices of soil organic matter decomposition were examined, particularly those relating to the ‘enzymic latch’ mechanism. The enzymic latch mechanism was stronger where the soil water content is high, for example the red mangrove soils and the mid salt marsh soils, which are frequently flooded, in line with the hypothesis. Analysis of decomposing mangrove leaf litter falls around these three species revealed much greater contributions of phenolics from the white and black mangrove species, whereas the red mangrove plants contributed comparably more nutrients. In the salt marsh, the mid tidal zone had the lowest rate of decomposition, despite the low zone being located closest to the sea, disproving the hypothesis. The mangrove encroached areas had more biological activity compared to the salt marsh area, suggesting that the colonisation of salt marshes with mangrove vegetation will have short-term negative impacts on below-ground carbon sequestration in coastal wetlands.